The Cellular Basis 179 



many parts which differ from one another in both structure and 

 function, and it is highly probable that there are enough of these 

 parts to provide a locus for every inheritance factor. 



There was a time when the cell was the Ultima Thule of biologi- 

 cal analysis and when the contents of cells were supposed to be 

 "perfectly homogeneous, diaphanous, structureless slime." Then 

 the nucleus was discovered within the cell, then the chromosomes 

 within the nucleus, then the chromomeres within the chromosomes, 

 and there is no reason to suppose that organization ceases with the 

 powers of our present microscopes. With every improvement of 

 the microscope and of microscopical technique, structures have 

 been found in cells which were undreamed of before, and it is 

 not probable that the end has been reached in this regard. We 

 know that cells contain nuclei and chromosomes and chromo 

 meres, centrosomes and plastosomes and microsomes, and we 

 know that some of these parts differ in function as well as struc- 

 ture. And there is no reason to doubt that if we had sufficiently 

 powerful microscopes we should find still smaller and smaller 

 units until we came at last to molecules and atoms. 



The fact that inheritance units from the two parents unite in 

 fertilization and later segregate in the formation of gametes, so 

 that the latter are pure with respect to any character, is a familiar 

 part of Mendelian inheritance (Fig. 59). Even if these units be 

 regarded as physiological processes they must be associated with 

 particular structures, since function and structure are inseparable 

 in life processes. What are the units in terms of cell structures 

 and where are they located in the cell? 



i. CHROMOSOMAL INHERITANCE THEORY. We have in .the 

 chromosomes, as Wilson especially has emphasized, an apparatus 

 which fulfils all the requirements of carriers of Mendelian fac- 

 tors (Fig. 60). Both factors and chromosomes come in equal 

 numbers from both parents; both maternal and paternal factors 

 and chromosomes pair in the zygote and separate in the gamete 

 as shown in Figs. 59 and 60; and so far as is known the chro- 



